In the prior art, there has been considerable interest in developing methods and apparatus for measuring the thickness of thin films on substrates. This need is particulary acute in the semiconductor manufacturing industry where extremely thin films are deposited on silicon substrates.
One technique which has been developed is classified as ellipsometry. In an ellipsometer, a probe beam, having a known polarization, is directed to reflect off the surface of the sample at an oblique angle. The thickness of the film layer on the substrate will effect the polarization state of the reflected beam. By measuring the polarization state of the reflected beam, information about the thickness of the layer can be derived.
Ellipsometers are quite useful for accurately measuring the thickness of very thin films. However, the spatial resolution of ellipsometers is limited to areas as large as 20-30 microns in diameter. In semiconductor manufacturing, it is often necessary to obtain thickness measurements in a much smaller region, on the order of one micron in diameter. Ellipsometers cannot fulfill this need. Another drawback with ellipsometers is that the range of measurement is quite narrow. While ellipsometers work quite well with extremely thin layers, they become ineffective at thicknesses greater than about 1000 Angstroms.
As noted above, ellipsometers rely on the measurement of the polarization state of a reflected probe beam. An entirely different class of layer thickness measurement devices found in the prior art rely on the interference effects (manifested in changes in surface reflectivity) created when a probe beam is reflected off the sample surface. Where a probe beam wavelength is selected which is at least partially transmitted by the thin film layer, reflections will occur both at the upper surface of the thin film layer and from the upper surface of the substrate. The interaction of the two reflected rays will create interference effects which vary based on layer thickness. These interference effects or changes in surface reflectivity can be detected by measuring the intensity of the reflected beam using a photodetector.
Spectrophotometers are one of the devices within this class of detectors. In a spectrophotometer measurement, the sample is scanned with different wavelengths of light. Since the phase shift through the film is different for each wavelength, each wavelength will undergo different interference effects. By studying the different interference effects created by the different wavelengths, information about the thickness of the thin film layer can be derived. In many spectrophotometers, the wavelength of the probe beam is varied over a significant range to reduce ambiguities in the measurement. Examples of such systems are disclosed in U.S. Pat. No. 3,824,017, issued July 16, 1974 to Gaylon, U.S. Pat. No. 4,293,224, issued Oct. 6, 1981 to Gaston, et al. and U.S. Pat. No. 4,555,766, issued Nov. 26, 1985 to Case, et al.
As described in the Gaston patent, interferometric devices can be used in situ to monitor changing layer thickness during processing. As can be appreciated, as the thickness of the layer changes during a coating operation, interference patterns in the reflected probe beam will change. By monitoring this change in interference patterns, information about the changing layer thickness can be obtained.
In the spectrophotometer techniques discussed above, known variables, such as changes in the wavelength of the probe beam or changes in the thickness of the layer are introduced to gain information about layer thickness. Another type of known variable which can be introduced is a change in the angle of incidence of the probe beam. Interference effects will vary as the angle of incidence the probe beam is changed. One example of a device where the angle of incidence of the input beam is varied can be found in U.S. Pat. No. 4,453,828, issued June 12, 1984 to Hershel, et al.
The latter techniques, which rely on the detection of interference effects, have better spatial resolution than the ellipsometry techniques discussed above. However, even the best interferometric techniques are limited to a resolution of about the 2-3 microns in diameter. More significantly, the interferometric techniques known heretofore are incapable of measuring thin films having a thickness less than 200.ANG.. Finally, the interferometric techniques are not nearly as accurate as the ellipsometer techniques discussed above.
A different interferometric technique is described in U.S. Pat. No. 4,660,980, issued Apr. 28, 1987 to Takabayashi, et al. In this device, a laser beam, which consists primarily of light having S-polarization, is reflected off the surface of the sample at close to Brewster's angle. Assuming the layer is relatively thick, i.e., on the order of 50 microns, a plurality of repeating minima or interference fringes will be produced across the reflected probe beam. In the Takabayashi device, the number of minima or fringes are counted and used to derive information about layer thickness. Takabayashi is therefore limited to measuring relatively thick thin film layers which will generate the multiple fringe effects. In addition, because Takabayashi forces the input beam to have an angle of incidence near Brewster's angle, tight focusing can not be achieved and spatial resolution must be sacrificed.
Accordingly, it would be desirable to provide a new and improved apparatus for measuring the thickness of thin films which is accurate over a wide range of thin film thicknesses.
It is another object of the subject invention to provide an apparatus for measuring thin film thicknesses within a region on the order of 1 micron or less in diameter.
It is still a further object of the subject invention to provide a new and improved apparatus for measuring the thickness of thin films from 50 to 50,000.ANG..
It is still a further object of the subject invention to provide a new and improved apparatus for measuring thickness of the thin films which utilizes only a single probe beam wavelength.
It is still another object of the subject invention to provide a new and improved apparatus for accurately measuring the thickness of thin film layers which does not require any means for actively changing the incidence angle of the probe beam.
It is still another object of the subject invention to provide a new and improved apparatus for measuring the thickness of a thin film layer which does not have to be operated in situ, where the thickness of the thin film is changing.
It is still a further object of the subject invention to provide a new and improved apparatus which is capable of determining the refractive index of a thin film as well as the thickness of the thin film layer.
It is a further object of the subject invention to provide a method for refining the accuracy of the thin film thickness measurement by providing an additional detector for recording the total power of the reflected probe beam.